1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor and a method of fabricating the same. More particularly, this invention is directed to a semiconductor acceleration sensor which has its silicon substrate subjected to three-dimensional processing to fabricate the formation of a weight portion or the like, and a method of fabricating the same.
2. Description of the Related Art
Conventionally, a semiconductor acceleration sensor of this type employs a wafer having an epitaxial layer formed on a silicon substrate, and has a weight portion and a cantilever formed by etching both surfaces of the wafer. In the method of fabricating the semiconductor acceleration sensor, first the bottom surface of the epitaxial layer is etched to the depth of the epitaxial layer to form half of a groove and the cantilever. Then the resultant structure is etched from the top surface only to form a groove region to permit penetration thereto, thus providing a weight portion.
FIG. 1 presents cross-sectional views exemplifying a step-by-step fabrication of a conventional semiconductor acceleration sensor. As shown in FIG. 1A, a conventional semiconductor diffusion process is performed on an epitaxial layer 2 epitaxially grown on a P-type silicon substrate 1. A gauge resistor 3 which detects acceleration, a pad 8 to be provided in a rim region 6A, and an aluminum wiring 11 which connects the gauge resistor 3 and the pad 8 are formed in a cantilever region 4A. Reference numeral "9" shows an oxide film and "7A" is a groove region. After an oxide film 13 is formed on the bottom surface of the P-type silicon substrate 1, etching is performed by an anodizing process to form an oxide film on a cantilever 4 and a groove portion 7, as shown in FIG. 1B. Since the silicon substrate 1 and the epitaxial layer 2 differ in a conductive type, the etching naturally stops at the bottom surface of the epitaxial layer 2. Further, as shown in FIG. 1C, etching restarts from the top surface of the N-type epitaxial layer 2 to complete a weight portion 5, a rim portion 6 around the weight portion 5, and the groove portion 7, thus providing a semiconductor acceleration sensor.
The above-described conventional semiconductor acceleration sensor involves problems in the structure of its weight portion or groove portion, so that dust or the like can easily enter that portion, deteriorating the sensitivity characteristics.
Further, according to the above conventional fabrication method, the overall process becomes complicated due to the following steps required in the etching process of the weight portion, groove portion or cantilever:
(1) Exposing and developing procedures have to be conducted twice in association with the bottom-surface and top-surface etching procedures.
(2) A jig and tools used in the top-surface etching procedure should be replaced when the bottom-surface etching is performed.
(3) The bottom surface of the silicon substrate should be covered with a protective film such as silicon oxide when the top-surface etching is performed after the bottom-surface etching.
The complicated process is likely to cause dust to enter the weight portion or groove portion during etching, resulting in accidents or yielding defects, and will increase the fabrication cost by the increased number of steps.